1. Field of the Invention
The present invention relates to oxide thin films produced by an ALD method. In particular, the present invention relates to yttrium-stabilised zirconium oxide (YSZ) thin films.
2. Description of Related Art
The continuous decrease in the size of microelectronic components leads to the situation in which SiO2 used today as the gate oxide in metal oxide semiconductor field effect transitions (MOSEFT) must be replaced with a higher permittivity oxide. This is due to the fact that in order to achieve the required capacitances, the SiO2 layer should be made so thin that the tunneling current would increase to a level affecting the functioning of the component. This problem can be solved by using a dielectric material having a higher dielectric constant than SiO2. For example, the capacitance of dynamic random access memory (DRAM) capacitors must remain nearly constant while their size decreases rapidly, and thus it is necessary to replace the previously used SiO2 and Si3N4 with materials which have higher permittivities than these and give higher capacitance density.
There is a number of materials exhibiting sufficiently high dielectric constant, but in addition to high permittivities, these dielectric thin films are required to have, among other things, low leakage current densities and high dielectric breakdown fields. The achievement of both of these properties presupposes a dense and flawless film structure. It is also important that the materials are stable in contact with silicon and can be exposed to the high post-treatment temperatures essentially without changes. Especially in the gate oxide application it is important that in the interface between silicon and the metal oxide having high dielectric constant there are very few electrically active states. In the memory application it is important that the structure of the dielectric of the capacitor is stable, since the temperatures used for activation of implanted ions are high.
Zirconium oxide, ZrO2 is an insulating material having a high melting point and good chemical stability. ZrO2 can be further stabilised by adding other oxides, the aim of adding other oxides is to eliminate the phase changes of ZrO2. Normally, the monoclinic crystal form is stable up to 1100° C. and tetragonal up to 2285° C., above which the cubic form is stable. The stabilisation is typically carried out by adding yttrium oxide (Y2O3), but also MgO, CaO, CeO2, In2O3, Gd2O3, and Al2O3 have been used. Previously, YSZ thin film layers have been produced, for example, by metal-organic chemical vapour deposition (MOCVD) (Garcia, G. et al., Preparation of YSZ layers by MOCVD: Influence of experimental parameters on the morphology of the film, J. Crystal Growth 156 (1995), 426) and e-beam evaporation techniques (cf. Matthée, Th. et al., Orientation relationships of epitaxial oxide buffer layers on silicon (100) for high-temperature superconducting YBa2Cu3O7-x, films, Appl. Phys. Lett. 61 (1992), 1240).
Atomic layer deposition (ALD) can be used for producing binary oxide thin films. ALD, which originally was known as atomic layer epitaxy (ALE) is a variant of traditional CVD. The method name was recently changed from ALE into ALD to avoid possible confusion when discussing about polycrystalline and amorphous thin films. Equipment for ALD is supplied under the name ALCVD™ by ASM Microchemistry Oy, Espoo, Finland. The ALD method is based on sequential self-saturating surface reactions. The method is described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811. The growth benefits from the usage of inert carrier and purging gases which makes the system faster.
When ALD type process is used for producing more complicated compounds, all components may not have, at the same reaction temperature range, an ALD process window, in which the growth is controlled. Mölsä et al. have discovered that an ALD type growth can be obtained when growing binary compounds even if a real ALD window has not been found, but the growth rate of the thin film depends on the temperature (Mölsä, H. et al., Adv. Mat. Opt. El. 4 (1994), 389). The use of such a source material and reaction temperature for the production of solid solutions and doped thin films may be found difficult when a precise concentration control is desired. Also the scaling of the process becomes more difficult, if small temperature changes have an effect on the growth process.
Mölsä et al. (Mölsä, H. et al., Adv. Mat. Opt. El. 4 (1994), 389) disclosed a process for growing Y2O3 by ALE-method. They used Y(thd)3 (thd=2,2,6,6-tetramethyl-3,5-heptanedione) as the yttrium source material and ozone-oxygen mixture as the oxygen source material in a temperature range of 400-500° C. As already discussed, no ALE window could be found since the growth rate increased steadily from 0.3 Å/cycle to 1.8 Å/cycle with increasing temperature.
Ritala et al. (Ritala, M. and Leskelä, M., Appl. Surf. Sci. 75 (1994), 333) have disclosed a process for growing ZrO2 by an ALD type process. ZrCl4 was used as the zirconium source material and water was used as the oxygen source material. The temperature in the process was 500° C. and the growth rate was 0.53 Å/cycle.